Photoacoustic wave is a kind of elastic wave that is generated in a thermoelastic process occurring when light in a wavelength range that is to be absorbed by the material is emitted to the material, and therefore, photoacoustic wave has attracted attention as a method for imaging absorption properties. Photoacoustic wave is also a kind of ultrasonic wave and characterized by low susceptibility to scattering compared with light. Accordingly, photoacoustic wave is employed as a method for in vivo imaging.
Photoacoustic microscopes that employ photoacoustic waves as detection signals for the purpose of imaging use, as excitation light, pulsed light corresponding to a wavelength range that is absorbed by an object to be observed. Such a photoacoustic microscope uses a technique of scanning in a specimen by a spot focused by an objective lens and detecting photoacoustic waves generated in different spots by means of a transducer or the like. According to the photoacoustic microscope, since, during the scanning in the specimen by a spot, the presence of an absorbing material in a focal point of the light generates a photoacoustic wave, the absorption properties in the specimen are imaged by detecting the generated photoacoustic wave.
Examples of such a photoacoustic microscope which are known include the one that is designed to improve spatial resolution. (Refer to, for example, Patent Literature 1.) In the above photoacoustic microscope, as FIG. 3 schematically illustrates a part thereof, excitation light L emitted from a laser pulse light source (which is not illustrated) passes through a condenser lens 101, a pin hole 102, an objective lens 103, a correcting lens 104, a triangular prism 105, silicon oil 106, a triangular prism 107, and an ultrasonic lens 108, and then, the excitation light L is focused into the specimen S. The triangular prisms 105 and 107 are coupled via the silicon oil 106. The specimen S is immersed in a liquid 110 in a liquid immersion bath 109. A photoacoustic wave U generated by the specimen S is collected by the ultrasonic lens 108 and enters the triangular prism 107, and then, reflected by an interface of the triangular prism 107 and the silicon oil 106 and detected by an ultrasonic transducer 111.
That is to say, in the photoacoustic microscope illustrated in FIG. 3, an excitation optical system and an ultrasonic wave guide system are coaxially arranged with use of the triangular prisms 105 and 107, and moreover, the objective lens 103 for focusing the light and the ultrasonic lens 108 for detecting an ultrasonic wave are arranged in a manner such that focal points thereof have a conjugate relation, so that the focused spot of the excitation light and the spot for detection of the ultrasonic wave are focused onto the same spot. With the above configuration, when, for example, a laser pulse light having a wavelength of 630 nm is used as the excitation light, an objective lens having a numerical aperture (NA) of 0.1 is used as the objective lens 103, and the excitation light is focused onto the specimen S with a diffraction limit of approximately 3.8 μm, an approximately 5-μm lateral resolution of photoacoustic wave is achieved.
However, since the photoacoustic microscope configured as illustrated in FIG. 3 detects a photoacoustic wave U with use of the ultrasonic transducer 111, when air is present in a wave guide for the photoacoustic wave U located between the specimen S and the ultrasonic transducer 111, the photoacoustic wave U is prevented from propagation. For the above reason, in the photoacoustic microscope illustrated in FIG. 3, it is required to immerse the specimen S and the ultrasonic lens 108 in the liquid 110 and to fill a liquid such as water and oil between adjacent elements disposed in the wave guide. The above requirements might result in limitation to the specimen S and a complicated configuration of a signal detection system.
On the other hand, the examples of the photoacoustic microscope also include the one that is capable of non-contact detection of a photoacoustic wave by utilizing vibration of a surface of a specimen that is caused when the photoacoustic wave generated inside the specimen in response to emission of excitation light propagates to the surface of the specimen. In the above photoacoustic microscope, a liquid such as oil is poured on the surface of the specimen or into the specimen, and detection light, which is different from excitation light, is focused onto a surface of the liquid so as to detect the detection light being modulated by the vibration of the surface of the specimen. As a method for the detection of modulated light, the Optical Coherence Tomography (OCT) (as described in Non-Patent Literature 1) and the light heterodyne method (as described in Non-Patent Literature 2) are known. Since the above photoacoustic microscope does not detect a photoacoustic wave directly but detects a photoacoustic wave by converting the photoacoustic wave into modulated light, the limitation to the specimen is significantly reduced, and the signal detection system may be configured in a simple manner.